1. Field of the Invention
The present invention relates to a semiconductor device and a method for manufacturing the semiconductor device. More particularly, the present invention relates to a semiconductor device that is suitable as a device including contact plugs having a barrier metal layer made of high-melting-point metal, and also to a method for manufacturing the semiconductor device.
2. Description of the Related Art
In most semiconductor devices, the interconnection layers lying above the silicon substrate are connected to the silicon substrate, by using contact plugs including a barrier metal layer. A conventional method for manufacturing a semiconductor device will be described, with reference to FIGS. 5A to 5E
First, as shown in FIG. 5A, boron is implanted into a specified surface region of a silicon substrate 11, at a dosage of 3×1015/cm2, thereby forming a p-type, heavily-doped diffused region (p+ diffused region) 14. Then, as depicted in FIG. 5B, an insulating film 15 made of silicon oxide (SiO2) is formed on the silicon substrate 11 that includes the p+ diffused region 14. Subsequently, a contact hole 17 is formed, which penetrates the insulating film 15 to reach the p+ diffused region 14.
Next, as illustrated in FIG. 5C, boron is implanted into a surface region of the p+ diffused region 14 at a dosage of 3×1015/cm2 and an acceleration energy of 5 keV. A p-type-impurity dosed layer 27 is thereby formed in the vicinity of the bottom of the contact hole 17.
Further, as shown in FIG. 5D, a contact metal layer 18 is formed in the contact hole 17 and on the insulating film 15. The contact metal layer 18 is made of titanium and has a thickness of about 10 nm. The resultant structure is subjected to a heat treatment for about one minute, in a nitrogen gas ambient, while maintaining the substrate 11 at about 700° C. As a result, a titanium silicide layer (not shown) is formed at the interface between the silicon substrate 11 and the contact metal layer 18.
Then, as depicted in FIG. 5E, a barrier metal layer 19 is formed, which is made of titanium nitride (TiN) and which has a thickness of about 10 nm. A tungsten film 20 is then formed on the barrier layer 19 including the internal of the contact hole 17. Subsequently, a CPM process is performed, by removing a part of the tungsten layer 20, a part of the barrier metal layer 19 and a part of the contact metal layer 18 from top of the insulating film 15, thereby leaving a contact plug 21 within the contact hole 17. Further, an upper interconnection layer 16 is formed on the insulating film 15 and on the contact plug 21 by using a method known in the art. A semiconductor device 200 having the structure shown in FIG. 5F is thus manufactured.
In the method described above, boron is used as a dopant to form the p-type-impurity dosed layer 27 in the p+ diffused region 14. The p-type-impurity dosed layer 27 is formed in order to prevent boron from diffusing from the p+ diffused region 14 into the contact metal layer 18 when the substrate 11 undergoes a heat treatment performed at about 700° C. If boron diffuses into the contact metal layer 18 during the heat treatment, the concentration of boron in the p+ diffused region 14 will decrease, inevitably increasing the contact resistance between the p+ diffused region 14 and the contact plug 21. The p-type-impurity dosed layer 27 can inhibit an increase in the contact resistance between the p+ diffused region 14 and the contact plug 21.
In recent years, semiconductor devices have become smaller and smaller and more highly integrated. Thus, the contact holes in the semiconductor devices have become smaller in diameter. Here arises a problem in that if the diameter of contact holes decreases to about 0.2 μm, the contact resistance can hardly be reduced. To reduce the contact resistance, p-type impurities may be implanted at such a high dosage as 1×1016/cm2, after the contact holes have been made, to thereby form a p-type-impurity region having a high impurity concentration. However, the resistance has not been decreased to a desired value in the conventional technique. A method for forming p-type-impurity regions having a high impurity concentration is described in, for example, Jpn. Pat. Appln. Laid-Open Publication No. 1-233726.